In PowhegControl, add MadSpin top quark decays to ttbb process
Requested in Jira ticket https://its.cern.ch/jira/browse/AGENE-1670 (in comments)
Native Powheg decays exhibit problems reported by @tpelzer (thanks!), so the MadSpin decays are desired as an alternative.
Also added: sanity checks to improve user friendliness and a small refactoring to improve maintainability.